This invention is concerned with the measurement and control of the partial pressure of a gas such as oxygen in gas-containing systems and is also concerned with improved transducers for use in gas flow control and other systems.
In life support systems, such as those associated with deep sea diving, exploration of outer space, and medical respiration applications, accurate measurement and control of the partial pressure of oxygen in the breathing atmosphere have been difficult to achieve, especially without undue complexity and weight. In order to overcome the disadvantages of prior systems which require amplification of the signal from an oxygen sensor in order to operate a solenoid oxygen admission valve, the applicant has previously proposed a system employing novel low-power electromechanical transducers for pneumatically controlling an oxygen admission valve directly in response to the signal output of a galvanic oxygen sensor. Such a transducer may vary the back pressure upon a servo valve controlling diaphragm by varying the gap between a jet nozzle and the periphery of a disc which is turned by an electric meter movement. When the applicant's prior system is employed in deep sea diving, for example, the flow of oxygen through the nozzle provides the diver's minimum metabolic uptake requirement, namely 0.3 standard liters per minute flow. In order to provide this low control flow through the nozzle, the gap between the nozzle and the disc must be set very close, that is, between 0.0003 and 0.001 inches. With the very small gap required for the minimum metabolic uptake, the disc may become jammed against the nozzle due to foreign particles, thermal expansion, or wear. The gap setting can be made larger, but the increase in oxygen flow can be dangerous if only one diver is breathing from the system and is resting or performing little work. The excess beyond the minimum metabolic uptake requirement could be bled to the outside of the system, but this would decrease the overall efficiency of the system and would produce escaping bubbles detrimental to convert use of the system. Moreover, the applicant's prior transducers have not been insensitive to the effects of varying ambient pressure in the system.
Electromechanical transducers proposed by others are even less satisfactory. For example, one such transducer employs the stagnation pressure produced in a receiver by a jet of gas from a nozzle. The stagnation pressure is varied by a surface which is attached to an electric meter movement and which enters the stream of gas laterally and gradually blocks the jet. Jet outlet pressures, and hence the transducer gain, are limited because of the reaction upon the surface, which is part of a long lever arm, and the power required to operate the transducer is higher than the available from low signal level sensors.